#1687: A faster (^)-function.
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Reporter: [EMAIL PROTECTED] | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.6.1
Severity: normal | Keywords:
Difficulty: Unknown | Os: Unknown
Testcase: | Architecture: Unknown
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This function performs better for me than the `(^)`-function in GHC. I
seem to only be able to test it for the Integer type though and its only
tested with ghc 6.6 (and ghc 6.6.1 by byorgey on #haskell).
I'm not sure if you really need this or if it is correct, but after
discussion on #haskell i was asked to make a bug report so here it is!
Enjoy. :)
{{{
module Pow (pow) where
import Prelude hiding ((^))
pow = (^)
(^) :: (Integral b, Num a) => a -> b -> a
x ^ y | y < 0 = error "Negative exponent"
| y == 0 = 1
| y == 1 = x
| odd y = x * x^(y - 1)
| otherwise = let x' = x^(y `div` 2)
in x' * x'
}}}
Tests
{{{
-- TestData.hs
module TestData where
e = 10^8
}}}
{{{
-- mytest.hs
import Pow
import TestData
main = print $ (2 `pow` e) `mod` 2
}}}
{{{
-- ghctest.hs
import TestData
main = print $ (2 ^ e) `mod` 2
}}}
Test results (performance)
{{{
$ time ./ghctest
0
real 0m11.744s
user 0m11.449s
sys 0m0.104s
$ time ./mytest
0
real 0m6.794s
user 0m6.696s
sys 0m0.084s
-}
}}}
QuickCheck test
{{{
-- qc.hs
-- $ ./qc
-- OK, passed 100 tests.
import Test.QuickCheck
import Pow
main = quickCheck prop
prop x y = y >= 0 ==> x `pow` y == x^y
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1687>
GHC <http://www.haskell.org/ghc/>
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